Phosphoenolpyruvate carboxykinase and its potential role in the catabolism of organic acids in the flesh of soft fruit during ripening

Abstract

Previous studies of grapes and tomatoes have shown that the abundance of phosphoenolpyruvate carboxykinase (PEPCK) increases in their flesh at the start of ripening, and that this coincides with a decrease in its citrate and/or malate content. Thus, PEPCK might function in the catabolism of organic acid anions during the ripening of these fruits. In the present study, the abundance of PEPCK was determined in the flesh of blueberries, raspberries, red currants, and strawberries at different stages of their development. In addition, changes in the amounts of citrate, malate, soluble sugars, isocitrate lyase, NADP-malic enzyme, phosphoenolpyruvate carboxylase, and pyruvate, orthophosphate dikinase in the flesh were determined. PEPCK was not detected in strawberry flesh, in which there was no dissimilation of malate or citrate. In the flesh of the other fruits, the abundance of PEPCK increased during ripening to an amount that was similar to that in grapes and tomatoes. In the flesh of blueberries and red currants, PEPCK was most abundant when there was dissimilation of malate. In the flesh of raspberries, PEPCK was most abundant when there was dissimilation of malate and citrate. These results are consistent with PEPCK playing a role in the dissimilation of citrate and/or malate in the flesh of these fruits during ripening. However, PEPCK was also present in the flesh of blueberries, raspberries, and red currants when there was no dissimilation of malate or citrate, and this raises the possibility that PEPCK might have additional functions. Dissection of blueberries provided evidence that both PEPCK and phosphoenolpyruvate carboxylase were present in the same cells, and possible functions for this are discussed.

Metabolic Profiling during Peach Fruit Development and Ripening Reveals the Metabolic Networks That Underpin Each Developmental Stage

“…5, B and C), support this theory (Fig. 7), particularly considering that a role for PEPCK in organic acid catabolism in certain fleshy fruits has been suggested previously during ripening (Famiani et al, 2005).…”

Alteration of the Interconversion of Pyruvate and Malate in the Plastid or Cytosol of Ripening Tomato Fruit Invokes Diverse Consequences on Sugar But Similar Effects on Cellular Organic Acid, Metabolism, and Transitory Starch Accumulation

“…In tomato, PEPCK abundance increases at the start of ripening, during which there is a decrease in malate content (Bahrami et al, 2001). This has led to the suggestion that PEPCK is involved in this dissimilation and that part of this dissimilated malate may be used in gluconeogenesis (Leegood and Walker, 2003;Famiani et al, 2005Famiani et al, , 2009. In contrast, NADP-ME isoforms function in chloroplasts and the cytosol (Drincovich et al, 2001;Gerrard Wheeler et al, 2005) and catalyze the reversible conversion of malate and pyruvate (Farineau, 1977;Drincovich et al, 2001).…”

Metabolic Engineering of Tomato Fruit Organic Acid Content Guided by Biochemical Analysis of an Introgression Line

“…Given this complexity and the variety of flux modes within carboxylic acid metabolism (Sweetlove et al, 2010), it is not obvious what the best strategy for engineering increased accumulation of carboxylic acids in fleshy fruits would be. This is reflected in the range of enzymes that have been proposed to control fruit citrate accumulation, including phosphoenolpyruvate carboxylase (Guillet et al, 2002), phosphoenolpyruvate carboxykinase (Famiani et al, 2005), citrate synthase (Sadka et al, 2000a), and aconitase (Sadka et al, 2000b; Degu et al, 2011).To identify a suitable metabolic engineering strategy for increasing carboxylic acid content of tomato, we undertook a detailed biochemical study of an introgression line of tomato (Eshed and Zamir, 1995). A specific line with an introgressed segment from Solanum pennellii on chromosome 2 (IL2-5) was identified that showed reproducible increases in citrate and malate during the later stages of fruit development but had minimal changes in gross developmental characteristics such as fruit size and number.…”

“…Given this complexity and the variety of flux modes within carboxylic acid metabolism (Sweetlove et al, 2010), it is not obvious what the best strategy for engineering increased accumulation of carboxylic acids in fleshy fruits would be. This is reflected in the range of enzymes that have been proposed to control fruit citrate accumulation, including phosphoenolpyruvate carboxylase (Guillet et al, 2002), phosphoenolpyruvate carboxykinase (Famiani et al, 2005), citrate synthase (Sadka et al, 2000a), and aconitase (Sadka et al, 2000b; Degu et al, 2011).…”